The present disclosure relates to a semiconductor-laser-device assembly including a semiconductor laser element and a semiconductor optical amplifier.
A laser device that generates pulsed laser light with a time width in the picosecond or femtosecond order is called ultrashort optical pulse laser device. In the following description, “laser light” represents pulsed laser light unless otherwise noted. The energy of light is concentrated in an extremely short period of time in case of laser light generated by such a laser device. Hence, the energy provides a high steepled-shape power (peak power), which may not be provided by continuous laser light. The laser light with the high peak power exhibits a nonlinear interaction with respect to a substance. Accordingly, application, which may not be provided by normal continuous laser light, is available. An example is application to a nonlinear optical effect. For specific example, application may be made on three-dimensional microscopic measurement or micromachining by using a multiphoton absorption effect.
In the past, a solid-state laser device represented by a titanium-sapphire laser device has been mainly used as an ultrashort optical pulse laser device. A solid-state laser device of related art frequently uses a large resonator as about 1 m. Also, another solid-state laser device has to be used for oscillating continuous laser light for excitation, resulting in low energy efficiency. In addition, it is difficult to keep mechanical strength for a large resonator, and maintenance expects professional knowledge.
As an ultrashort light pulse laser device that addresses the problems of the solid-state laser device, a semiconductor laser element, which uses a semiconductor as a gain medium, is developed. Since a semiconductor is used, the resonator can be easily decreased in size. Also, mechanical stableness can be easily attained as the result of the decrease in size. Accordingly, maintenance, which expects high technical skill, can be facilitated. Also, since a semiconductor can be directly excited by current injection, the energy efficiency can be increased.
There is provided a mode-locking method as a method of generating laser light with a time width of about several picoseconds by using a semiconductor laser element. The mode-locking method includes active mode-locking that modulates a gain or a loss with the same period as a periodic time of a resonator, and passive mode-locking that enables operation by providing an element indicating a nonlinear optical response such as a saturable absorber in a resonator. Among these methods, the passive mode-locking is suitable for generating laser light with a time width of about several picoseconds. To generate an ultrashort pulse by the passive mode-locking, a saturable absorber is provided typically in a laser resonator. In a mode-locked semiconductor laser element based on the passive mode-locking (hereinafter, merely referred to as “mode-locked semiconductor laser element”), a p-side electrode of the mode-locked semiconductor laser element is divided into a gain portion and a saturable absorption portion (SA portion). Such a mode-locked semiconductor laser element is called “multi-electrode mode-locked semiconductor laser element.” When a forward bias current flows to the p-side electrode in the gain portion, a gain of a laser oscillator is generated. In contrast, when a reverse bias voltage is applied to the p-side electrode of the saturable absorption portion, the saturable absorption portion is operated as a saturable absorber. Then, when the reverse bias voltage is controlled, a recovery time for saturated absorption can be adjusted. Accordingly, the pulse width of laser light to be generated can be controlled. As described above, the multi-electrode mode-locked semiconductor laser element has an advantage in which the gain portion and the saturable absorption portion can be electrically controlled. The saturable absorption portion is made of a gain medium in a waveguide. Hence, as the result of that laser light is efficiently confined in the waveguide, absorption can be easily saturated with small energy. Therefore, it is difficult to obtain a large output with the multi-electrode mode-locked semiconductor laser element.
In this situation, a system called “master oscillator power amplifier (MOPA)” is effective. MOPA amplifies an ultrashort pulse output from the mode-locked semiconductor laser element by using a semiconductor optical amplifier (SOA). The semiconductor optical amplifier mentioned herein does not convert an optical signal into an electric signal, but directly amplifies light. The semiconductor optical amplifier has a laser structure from which a resonator effect is eliminated as possible, and amplifies incident light based on an optical gain of the semiconductor optical amplifier. With this method, laser light with a time width of several picoseconds emitted from the mode-locked semiconductor laser element is amplified by the semiconductor optical amplifier, and hence the pulse energy is increased.